A light emitting diode (LED) chip is a junction type electroluminescent semiconductor device that converts an electrical signal to a light signal. Using gallium nitride based LEDs as a solid light source, with the advantages of small sizes, high efficiency, long life-span, low energy consumption, and environmental friendliness, is regarded as a revolution in the illumination history, and becomes the research and industrial focus in the international semiconductor and illumination fields.
III-V group nitride materials, such as GaN, InGaN, AlGaN and AlGaInN, have a continuously adjustable direct bandwidth ranging from 0.7 eV to 6.2 eV, which covers the spectrum range from ultraviolet light to infrared light. They are suitable materials for making a blue, green or white light emitting device.
Sapphire substrates are one type of the conventional substrates for growing a GaN layer through hetero-epitaxy technology. Because there is a large mismatch of lattice constants and difference in thermal expansion coefficients between the sapphire substrate and the GaN epitaxial layer, great stress and a plurality of crystal defects may exist in the GaN epitaxial layer. And the crystal defects may become nonradiative recombination centers, which will affect the internal quantum efficiency and the performance of the LED. FIG. 1 is a cross-sectional view of a conventional LED. As shown in FIG. 1, a gallium nitride based LED may comprise: a sapphire substrate 10; a semiconductor structure 20 formed on the sapphire substrate 10 and comprising an n-type gallium nitride layer 21, a light emitting layer 22 formed on the n-type gallium nitride layer 21, and a p-type gallium nitride layer 23 formed on the light emitting layer 22; a transparent conductive layer 40 formed on the p gallium nitride layer 23; an n-type electrode 30 formed on the n-type gallium nitride layer 21; and a p-type electrode 50 formed on the transparent conductive layer 40. The n-type electrode 30 and the p-type electrode 50 may be connected with a power supply. A current may diffuse in the transparent conductive layer 40 and pass through the light emitting layer 22 in a vertical direction, and the light emitting layer 22 may generate and emit light. However, because a plurality of crystal defects may exist in the semiconductor structure 20, electrons and electron holes may recombine in the crystal defects, and energy may be released in the form of heat rather than light, forming a nonradiative recombination. If the semiconductor structure 20 is formed on a flat sapphire substrate 10, crystal defects may be even increased in the semiconductor structure 20, which may cause more nonradiative recombinations to be generated, resulting in increased heat production and decreased energy utilization rate. This will further reduce the internal quantum efficiency and the performance of the LED.